1. Field of the Invention
The present invention relates to a contactless communication medium that performs contactless radio communication with a nearby reader/writer, an antenna pattern-placed medium included in the contactless communication medium, a communication apparatus in which the contactless communication medium is built in, and an antenna adjusting method applied to contactless radio communication.
2. Description of the Related Art
Contactless communication media called contactless IC cards are in widespread use as contactless communication media for performing contactless radio communication with a nearby reader/writer. For example, such contactless IC cards are widely used for railway ticket gate systems, bill payment systems for convenience stores, and entrance and exit control systems. Such contactless IC cards are also called radio frequency identification (RFID) or radio IC tags.
Such contactless IC cards come with an embedded IC chip, allowing for quick response and processing for purposes such as management of entrance and exit, billing, and so on. Thus, contactless IC cards are of very high utility in comparison to magnetic cards or the like.
FIGS. 8A and 8B show an example of the configuration of a contactless IC card according to the related art. FIG. 8A shows a state in which the circuit for contactless communication is placed on a resin base. A contactless IC card as the actual product has a film or the like as an outer covering material placed on its surface so that the internal circuit is hidden.
The configuration shown in FIG. 8A will be described. On the front surface of a base 10, an antenna coil section 20 is placed at a location near the outer perimeter of the base 10. The antenna coil section 20 is formed by winding a conductor pattern of a predetermined width made of a conductor such as copper or aluminum a plurality of times (about four times in this example), and placing the windings at a predetermined interval, on the front surface near the outer perimeter of the base 10.
One end 21 and the other end 22 of the antenna coil section 20 are connected to an IC chip 11, which is an integrated circuit component that performs communication processing. In this case, the one end 21 of the antenna coil section 20 is brought into electrical continuity with the back side of the base 10, and is connected to the IC chip 11 that performs communication processing, via a conductor pattern 14 on the back side. The other end 22 of the antenna coil section 20 is connected to the IC chip 11 via a conductor pattern 13.
The one end 21 and the other end 22 of the antenna coil section 20 are connected to a capacitor 12 and an adjusting capacitor 30. The capacitor 12 and the adjusting capacitor 30 are also connected by using the conductor pattern 14 on the back side.
The capacitor 12 is used to store electric charge generated by a carrier wave received by the antenna coil section 20, and obtain electric power for driving the IC chip 11. The capacitor 12 includes a first electrode section formed by a conductive pattern on the front side, and a second electrode section formed by a conductive pattern on the back side. The capacitor 12 stores electric charge on the first electrode section and the second electrode section that are opposed to each other via the base 10. Each of the electrode sections forming the capacitor 12 has a relatively large area so as to enable storage of relatively large electric charge.
The adjusting capacitor 30 is used for the purpose of changing resonant frequency. The adjusting capacitor 30 includes a first conductor pattern 31 on the front side which is connected to the other end 22 of the antenna coil section 20, and a second conductor pattern 32 on the back side which is connected to the conductor pattern 14. The first conductor pattern 31 on the front side is placed in comb-tooth form, and the second conductor pattern 32 on the back side is placed so as to orthogonally intersect the comb-toothed portion. Electric charge is stored at their orthogonal intersections. The adjusting capacitor 30 is a small capacitance capacitor in comparison to the capacitor 12. The adjusting capacitor 30 is provided for the purpose of cutting off the comb-toothed conductor pattern partway to reduce the capacitor's capacitance when adjusting resonant frequency during the manufacturing process of the contactless IC card, thereby raising resonant frequency.
FIG. 8B shows an equivalent circuit of the configuration of the contactless IC card shown in FIG. 8A.
As shown in FIG. 8B, the IC chip 11, the capacitor 12, and the adjusting capacitor 30 are connected in parallel to the antenna coil section 20.
An adjustment process to raise resonant frequency with the adjusting capacitor 30 is performed by cutting off the first conductor pattern 31 and the second conductor pattern 32 partway. This process is performed by, for example, boring a hole all the way through the base 10 at the cutting location of the first conductor pattern 31, and drawing out the first conductor pattern 31 or the second conductor pattern 32.
This adjustment process of resonant frequency during the manufacturing process is performed automatically using an adjusting apparatus (not shown). The adjusting apparatus is configured to previously hold data on the cutting position for correcting the resonant frequency of the communication medium, determine the cutting position on the basis of the actually measured resonant frequency, and adjust the resonant frequency by boring a hole in the base at the determined position. Through this adjustment, a contactless IC card with an appropriate resonant frequency can be provided.
FIGS. 9A and 9B show an example of configuration with a center tap, different from the example shown in FIGS. 8A and 8B.
The configuration shown in FIG. 9A will be described. On the front surface of the base 10, the antenna coil section 20 formed by winding a conductor pattern a plurality of times is placed at a location near the outer perimeter of the base 10. The one end 21 and the other end 22 of the antenna coil section 20 are connected to the IC chip 11, which is an integrated circuit component that performs communication processing. The one end 21 of the antenna coil section 20 is connected to the IC chip 11 that performs communication processing, via the conductor pattern 14 on the back side.
On the back side, the capacitor 12 is connected to the one end 21 of the antenna coil section 20. On the front side, the capacitor 12 is connected to an end 24 of an antenna extension 23 that is extended from the other end 22 of the antenna coil section 20.
For the adjusting capacitor 30 as well, the conductor pattern 14 on the back side is connected to the second conductor pattern 32, and the end 24 on the front side is connected to the first conductor pattern 31.
FIG. 9B shows an equivalent circuit of the configuration of the contactless IC card shown in FIG. 9A.
As shown in FIG. 9B, the IC chip 11 is connected to the antenna coil section 20, and the capacitor 12 and the adjusting capacitor 30 are connected via the antenna coil section 20 and the antenna extension 23. The other end 22 that is the connecting point of the antenna coil section 20 and the antenna extension 23 serves as a center tap. The adjustment process with the adjusting capacitor 30 is the same as that in the example shown in FIGS. 8A and 8B.
In the case of the configuration shown in FIGS. 9A and 9B, by making an adjustment using the adjusting capacitor 30, it is possible to change the overall inductance value without changing the value of inductance connected to the IC chip 11. In the case of the example shown in FIGS. 9A and 9B as well, an adjustment to raise resonant frequency is made.
Japanese Unexamined Patent Application Publication No. 2003-67693 describes about a configuration for performing communication using a contactless IC card.